1. Field of the Invention
This invention relates to a device for limiting the direction of rotation of a synchronous motor to rotate the same in one direction.
2. Prior Art
The direction of rotation of a synchronous motor is not predetermined because of its electrical characteristics. Therefore, a device or mechanism for limiting the rotational direction of the sychronous motor is required in those applications in which a rotational direction of the motor is preset.
Japanese Laid-Open (Kokai) Utility Model Application Nos. 38863/86 and 41369/86 discloses such rotational direction-limiting mechanisms in which a reverse rotation-preventing lever is rotatably mounted on and frictionally engaged with a shaft of a first gear of a gear train, connected to an output shaft of a synchronous motor, so that this lever is rotatable about the axis of the first gear, the first gear being in mesh with a rotor shaft. A friction torque is applied between the lever and the first gear by means of either a viscous resistance produced by a grease or spring-biased contact. When the rotor is rotated in a reverse direction, the distal end of the reverse rotation-preventing lever is brought into striking engagement with an abutment surface, formed on the rotor, thereby to prevent a reverse rotation of the rotor and to urge the rotor to rotate in its normal direction by a reaction force produced upon engagement of the lever with the abutment surface.
With this conventional arrangement, however, depending on the strength of the transmitting force by the viscous resistance of the grease or the spring bias, the reverse rotation-preventing lever sometimes fails to properly rotate with the first gear, so that the distal end of this lever is not properly engaged with the abutment surface, thus failing to prevent the reverse rotation. Further, even if the rotor is successfully controlled to rotate in its normal direction, the friction torque between the first gear and the reverse rotation-preventing lever always constitute a load on the part of the small synchronous motor, so that the motor torque is not efficiently used.
Korean Utility Model Publication No. 86-3241 discloses another rotational direction-limiting mechanism in which a leaf spring is brought into abutting engagement with a projection, formed on a first gear of a gear train, when the rotor is rotated in a reverse direction thereby to prevent the reverse rotation and also to urge the rotor in its normal direction. The leaf spring is so designed as to be flexed to allow the rotation of the rotor in its normal direction, and therefore the leaf spring needs to be pressed against the first gear. This gives rise to wear and an undesirable load on the motor. Another problem is that if the force under which the leaf spring is held against the first gear is low, the leaf spring may fail to properly prevent the reverse rotation of the rotor.